A half transparent or hybrid type liquid crystal display (LCD) device has recently come into wide use. Such a half transparent type LCD device includes both optically reflective and transparent electrodes per pixel which are supposedly suitable for outdoor use. The reflective electrodes reflect ambient incident light as a light source for display while the transparent electrodes are prepared to make use of light from a rear light source for such a half transparent type LCD device.
FIGS. 5 and 6 are a schematic layout in a sectional view of a half transparent type LCD device and its schematically sectional view cut along the line VI—VI in the LCD device shown in FIG. 5, respectively. Major components of the LCD cell will be explained below with reference to FIGS. 5 and 6, and the other components understood by those skilled in the art are omitted for the sake of simplicity.
An LCD cell includes a thin film transistor (TFT) array substrate 10, a counter substrate 22 and a liquid crystal (LC) layer 28. The counter substrate 22 is provided opposite to the TFT array substrate 10. The LC layer 28 is held between the TFT array and counter substrates 10 and 22. The TFT array substrate 10 is formed on a transparent glass substrate 11 and is divided into display and non-display (or frame) areas 31 and 32. The display area 31 includes signal lines 12, an interlayer insulation film 13 and scanning lines 14 (not shown in FIG. 6). The signal lines 12 cross the scanning lines 14 with the interlayer insulation film 13 located between them. The signal and scanning lines 12 and 14 are disposed in a matrix form. Such a matrix has elements where pixels are provided with pixel TFTs and electrodes 15 and 16. Auxiliary capacitor lines 17 are provided in parallel with the scanning lines 14.
Each pixel electrode 16 has transparent and reflective portions 33 and 34 in its central and peripheral portions, respectively. The transparent portion 33 of the pixel electrode 16 used as a transparent electrode 18 is made of an indium-tin-oxide (ITO) film. The reflective portion 34, however, is covered with a metal film used for a reflective electrode 19. Since there are many signal and scanning lines 12 and 14 in the peripheral portion, it is advantageous to provide the central portion with the transparent electrode 18 in terms of light utilization efficiency. The reflective electrode 19 has an uneven surface to widen viewing angles for reflecting light. On the outer surface of the TFT array substrate 10 a polarizer 20 and a rear light source 21 are disposed in that order. The inner surface of the TFT array substrate 10 is covered with an alignment film, not shown.
The counter substrate 22, on the other hand, also includes a transparent glass substrate 23 on which a counter electrode 24, a color filter 25, etc. are formed in that order. The inner surface of the counter substrate 22 is covered with an alignment film, not shown, either. A polarizer 26 is put on the outer surface of the counter substrate 22.
The TFT array and counter substrates 10 and 22 are opposite to each other at a predetermined distance by column like spacers 27 and are fixed together by a sealant provided at their peripheral portions. A liquid crystal material which becomes the LC layer 28 is injected between the TFT array and counter substrates 10 and 22.
Meanwhile, the transparent and reflective portions 33 and 34 of the half transparent type LCD device have different light transmission paths through the LC layer 28. Light L1 from the rear light source 21 passes through the LC layer 28 once with respect to the transparent portion 33. Ambient light incident on the counter substrate 22 passes through the LC layer 28, is reflected from the reflective electrode 19, passes through the LC layer 28 again and is derived from it as reflected light L2. In short, the optical path for the reflected light L2 to pass through the LC layer 28 is much longer than that for the light L1 to do so. In order for both reflective and transparent displays to obtain an optimal optical property, optimum cell gaps should be designed for the transparent and reflective portions 33 and 34, respectively. Thus, as shown in FIG. 6, a convex portion 29 made of a resin film is provided under the reflective electrode 19 and a concave portion 30 is also made at the transparent electrode 18. As a result, the cell gap at the reflective portion 34 is made smaller than that at the transparent portion 33 so that the optical path for the ambient light to pass through the LC layer 28 can be adjusted. An LCD device of this sort is generally called a half transparent type LCD device with multiple gaps.
Column-like spacers 27 provided on the TFT array substrate 10 have been becoming the mainstream to maintain the cell gap because they are a much easier way of controlling the cell gap than ball spacers and cause uniform display quality without any substantial light leakage. In addition, such column-like spacers 27 are disposed not only on the display area 31 but also on the frame area 32 to maintain the cell gap for an entire panel of the LCD device.
Where the column-like spacers 27 are provided on the TFT array substrate 10, it is necessary to form them on the resin insulation layer because their firm fixation and density must be secured. The column-like spacers 27 are ordinarily formed on auxiliary capacitor lines 17 in the display area 31 as shown in FIG. 5 to avoid affecting pixel aperture rates. Thus, entirely convex portions are provided on the auxiliary capacitor lines 17 and the column-like spacers 27 are formed on the convex portions. Similarly, a resin layer used as a base member is formed in the frame area 32 and column-like spacers 38 are formed on the resin layer to maintain a uniform gap.
The column-like spacers 27 made of a resin material are formed by the steps of coating a photoresist film, exposure, development and removal. In the step of coating a photoresist film, unevenness on a surface of the film produces a great influence on the thickness of the coated film. It is called a “leveling” phenomenon when concave portions in the unevenness of the coated film make their film thickness thinner than flat portions. The frame area 32 is generally flat but the transparent and reflective portions 33 and 34 in the display area 31 cause the unevenness or such a “leveling” phenomenon. Thus the thickness of a coated photoresist film in the display area 31 is thinner than that in the frame area 32, so that the height of column-like spacers in the former are different from that in the latter. In other words, the cell gap in the display area 31 is not necessarily constant or uniform, so that it leads unsatisfactory display quality for the LCD display.